1. Field of the Invention
The present invention relates to a high-frequency amplifier comprising an amplifying transistor, and a bias circuit that supplies a bias current to the base of the amplifying transistor and having a temperature compensation function.
2. Background Art
In a high-frequency amplifier circuit of transmission power for a mobile phone, a bipolar transistor is used, and to the base thereof, a bias circuit for supplying a bias current is connected. Since small change in the properties of the high-frequency amplifier circuit due to temperature change is desired, the bias circuit is provided with a temperature compensation function.
FIG. 4 is a circuit diagram showing a conventional high-frequency amplifier circuit that has a bias circuit having a temperature compensation function. The conventional high-frequency amplifier circuit has an amplifier transistor 1 for amplifying high-frequency signals, and a bias circuit 2 for supplying a bias current to the base of the amplifier transistor 1.
Here, amplifier transistor 1 has a collector connected to an amplifier transistor collector power terminal 3, and an emitter connected to a grounding point. Bias circuit 2 also has a reference voltage input terminal 4 wherein a reference voltage is input from an external source, a first transistor 5, resistors 6 and 7, a first transistor connected as a diode (referred to here as a transistor diode) 8, and a second transistor diode 9.
The collector of the first transistor 5 is connected to a power terminal 10 for the bias circuit, the base thereof is the reference voltage input terminal 4 through the resistor 7, and the emitter thereof is connected to the base of the amplifier transistor 1.
The resistor 6 is connected between the connecting point of the emitter of the first transistor 5 to the base of the amplifier transistor 1 and the grounding point. The first transistor diode 8 and the second transistor diode 9 are serially connected to each other, and are connected between the connecting point of the base of the first transistor 5 to the resistor 7 and the grounding point.
The first transistor 5 and the resistor 6 constitute an emitter-follower circuit to be able to supply a sufficient base current to the amplifier transistor 1. The first transistor diode 8 and the second transistor diode 9, which constitute a temperature compensation unit, can compensate the temperature characteristics of the idle current Ic of the amplifier transistor 1. The level that can be compensated depends largely on the supplied reference voltage, and the higher the reference voltage, the smaller change in the idle current Ic due to temperature change.
If AlGaAs HBTs (hetero-junction bipolar transistors) having a forward base-emitter voltage of about 1.2 volts are used as the first transistor diode 8 and the second transistor diode 9, the base voltage of the first transistor 5 becomes about 2.4 volts. In general, a voltage of 3 volts is used as the reference voltage.
In order to reduce change in the idle current of the amplifier transistor 1 due to temperature change, the current of the first transistor 5 must be constant regardless of change in the base voltage V1 of the first transistor 5 due to temperature change. It is important for this to make the current of the first transistor 5 constant; that is, it is important to make the current flowing in the resistor 7 constant regardless of change in V1. In this case, since the current flowing in the resistor 7 is in proportion to the potential difference between the reference voltage and V1, it is required to raise the reference voltage as high as possible (3 volts or more). Thereby, mobile phones had problems that made low-voltage operation difficult, increased the number of parts, and increased consumed current.
The conventional high-frequency amplifier circuit had also a problem that change in the impedance of the bias circuit 2 viewed from the amplifier transistor 1 significantly affected the high-frequency characteristics of the high-frequency amplifier circuit. Especially, in a high-frequency amplifier circuit used in CDMA modulation and the like that require a high linearity, change in the impedance of the bias circuit must be small relative to the change in the operation of the bias circuit 2 due to change in the reference voltage. There was also another problem that the accuracy of the reference voltage of about ±1% was required to minimize the fluctuation of the idle current of the amplifier transistor 1.
There was another problem that the properties of the high-frequency amplifier circuit, such as distortion and noise power in Rx band, were deteriorated by the effect of low-frequency impedance.
To solve the above-described problems, the first object of the present invention is to provide a high-frequency amplifier circuit that can compensate change in the idle current of the amplifier transistor due to temperature change without raising the reference voltage, and can avoid the unstable operation of high-frequency signals due to feedback or the like. The second object of the present invention is to provide a high-frequency amplifier circuit that can minimize change in impedance of the bias circuit due to change in the operations, and can minimize the fluctuation of the idle current of the amplifier transistor due to the fluctuation of the reference voltage. The third object of the present invention is to provide a high-frequency amplifier circuit that can improve the properties of the high-frequency amplifier circuit, such as distortion and noise power in Rx band.